1. Field of the Invention
This invention relates to a device for detecting the position of an interface between a melt and a monocrystalline rod. This device is used in a monocrystal producing apparatus based on the floating zone (hereinafter referred to as FZ) method.
2. Description of the Prior Art
In general, in a monocrystal producing apparatus based on the FZ method, such as that shown in FIG. 8, a floating zone 16 is formed by an induction heating coil 14 between a raw-material rod 10 and a monocrystalline rod 12 while these rods are relatively moved downward. In a conventional method of this type, a diameter Ds of the crystalline rod 20 at the crystallizing interface is detected by an industrial TV camera 18, and the apparatus is controlled by adjusting the detected diameter to a set value. The inventor of the present invention has found that a diameter Dm of a cross-section of a melt shoulder portion 21 at a predetermined distance from the cystallization interface 20 strongly correlates to the diameter Ds with a certain time interval. Also, this finding has enabled the control of the crystal diameter Ds at the crystallizing interface to be performed with improved response and stability. Specifically, in the process of forming a conical portion, it is possible to minimize the axial length while preventing spillover of the melt (U.S. application Ser. No. 07/132, 191, EP application No. 8711849.2).
In this method of controlling the diameter Dm of a melt shoulder portion, the value of the diameter Dm of a melt shoulder portion predicts the value of the diameter Ds of the crystal rod at the crystallizing interface to be obtained after a predetermined time period has passed, and the diameter Dm should be accurate enough to measure in order to precisely estimate the value of the diameter Ds. Beyond a critical value of the diameter Dm, which depends on the diameter Ds and the magnitude of the surface tension of the melt, the spilling of the melt over the crystal edge occurs, and the due control is therefore specially important in the conical portion zoning when the melt must bulge over the edge of the crystallizing lower portion.
In a conventional system, such as the one disclosed in Japanese Patent Publication No. 48424/1972, the floating zone 16 and portions therearound are observed by an industrial TV camera 18, a luminance signal which represents the luminance along a sensing line L is obtained by scanning from the monocrystalline rod 12 to the floating zone 16, and a position at which the level of the luminance signal abruptly decreases is located as the position of the crystallization interface 20.
The level of this luminance signal ordinarily changes as indicated by the line (X) in FIG. 7. The arrow B in FIG. 7 indicates the direction of scanning in accordance with the conventional method.
However, crystal growth lines are formed on the side surface of the monocrystalline rod 12 in accordance with the crystallographic orientation, and the crystalline interface profile is inconsistently deformed downward at a position at which the crystal growth line intersects the crystallization interface 20, thereby forming a crystal growth curve 22. The luminance signal level thereby changes as indicated by the line (Y) in FIG. 7. There is therefore a possibility of an error occurring in determination of the position of the crystallization interface 20 by a distance d.
Thus, in this detection method, the error makes inevitably the positioning of a melt shoulder portion for measurement inaccurate, and it is therefore not possible to detect a necessary diameter Dm of a melt shoulder portion constantly and accurately.